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IRQ Queery

Discussion in 'A+' started by robbo1962, Aug 24, 2006.

  1. robbo1962

    robbo1962 Byte Poster

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    Not sure if i am correct in my understanding of this but here goes. I believe that it is possible to run several USB devices using the same IRQ, if this is so could someone please explain how it is done without having conflicts?
     
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  2. Mr.Cheeks

    Mr.Cheeks 1st ever Gold Member! Gold Member

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    USB only takes 1 IRQ port.
    All the rest of the usb's that you connect are assigned to that same IRQ. you are allowed up to 127 USB devices (daisy chained) to connect.

    With the conflict error i cant remember why, but i know its straightforward.

    Thats what i know anyway, plus have not got my MM book on me, so im aure someone will add to what i have written.
     
  3. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    The IRQ is for the USB root controller. This handles all the actual devices on the bus. This is one reason why only one IRQ is required.

    Harry.
     
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  4. robbo1962

    robbo1962 Byte Poster

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    could you expand a little on how that works H?, i can't find anything about it in MM
     
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  5. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    USB is a bus system (the B in the acronym). On the PC end of the bus is a controller, which controls (surprise) the bus and organises all the devices on the bus. All the devices have to go through the controller to send data to the PC.

    This means that the only thing that requires an IRQ is the controller, hence only using one.

    IDE is similar - only one IRQ is used for the controller - which in turn controls up to two drives. SCSI is another example - the controller uses one IRQ while controlling up to 15 devices.

    Harry.
     
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  6. zimbo
    Honorary Member

    zimbo Petabyte Poster

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    ok im not 'H' but im just as good.... here is a very watered down example....

    Lets say 4 people live in a house... and that house is 12 Blue Street... for all 4 people the postal address is 12 Blue street and if another 100 people came to live in the house their postal address would also be 12 Blue street...

    Same with an IRQ... the USB root controller (house) has an IRQ (12 Blue Street) and the devices you connect to it are the people... where i think you getting confused is you saying i have 6 USB ports lets say (3 on the back of my case and 3 on the front) how can i share an IRQ? Well as you can see its the USB root controller that *holds* the IRQ...
     
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  7. robbo1962

    robbo1962 Byte Poster

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    A quality explaination Harry, did'nt think of it as being along the same lines as IDE and SCSI thanks robbo
     
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  8. Neil

    Neil Byte Poster

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    Ok I learnt somn there........its the USB "CONTROLLER" that occupies the IRQ........thanks guys! :oops:
     
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